1. Technical Field
This invention relates to an aluminum wire press-clamping terminal (or crimp-type terminal) for being press-clamped (or crimped to) an aluminum wire having a conductor portion in which a plurality of aluminum- or aluminum alloy-made strands are twisted.
2. Background Art
Generally, copper wires are used in a wire harness installed in a vehicle such as an automobile. For connecting such wire harnesses together or for connecting such a wire harness to on-board equipments, terminals are fixedly secured to the copper wires, and the terminal of this kind is fixedly secured to the copper wire usually by press-clamping (that is, crimping).
The terminal for being press-clamped to the copper wire is formed by blanking a piece of a predetermined shape from an electrically-conductive sheet (made, for example, of a copper alloy) into a predetermined shape and then by bending the blanked-out piece into a final shape. Typically, such a terminal includes a bottom plate portion for placing a conductor portion (composed of a plurality of copper strands twisted together) of the copper wire thereon, and a pair of caulking piece portions extending from the bottom plate portion so as to hold the conductor portion (placed on the bottom plate portion) therebetween. When the caulking piece portions are caulked to the conductor portion, each caulking piece portion embraces part of the strands of the conductor portion, with its distal end portion piercing into the conductor portion. Thus, the terminal is press-clamped to the conductor portion of the copper wire.
In recent years, in view of the shortage of copper resources, a lightweight design of a vehicle and recyclability, attention has been directed to aluminum wires. However, an oxide film formed on a surface of an aluminum product is thicker than that on a copper product, and in the case of an aluminum wire, a contact resistance between a conductor portion and a terminal tends to become relatively high. One known method of reducing this contact resistance is to caulk the caulking piece portions of the terminal hard to the conductor portion, thus increasing the compressibility of the conductor portion. In this method, oxide films formed on the strands of the conductor portion are destroyed, so that the contact resistance between the conductor portion and the terminal is reduced. In the present specification, “the compressibility of the conductor portion” is defined by the ratio of the cross-sectional area of the conductor portion after the press-crimping to the cross-sectional area of the conductor portion before the press-clamping”.
However, stresses acting on the conductor portion increases with the increase of the compressibility of the conductor portion. And besides, aluminum is inferior in mechanical strength to copper. Therefore, when an excessive stress acts on the conductor portion of the aluminum wire, a press-clamping strength of the terminal is greatly lowered. Therefore, there has been proposed an aluminum wire press-clamping terminal of the type in which a contact resistance between the reduction of a contact resistance between a conductor portion and the terminal and the securing of a press-clamping strength of the terminal are both achieved (see, for example, JP-A-2007-173215).
As shown in FIG. 5, the press-clamping terminal disclosed in JP-A-2007-173215 has a plurality of grooves 106 formed in inner surfaces (contact surfaces) of a bottom plate portion 104 and caulking piece portions 105 which are adapted to contact a conductor portion 102. A depth of these grooves is determined according to a diameter of each of strands forming the conductor portion 102 of an aluminum wire 101. When the caulking piece portions 105 are caulked onto the conductor portion 102 of the aluminum wire 101, the strands 103 of the conductor portion 102 intrude into the grooves 106, so that oxide films formed on the surfaces of the strands 103 are destroyed, and besides the withdrawal of the conductor portion 102 is prevented. By doing so, a contact resistance between the conductor portion 102 and the terminal is reduced, and also a press-clamping strength of the terminal is secured.
At the time when the caulking piece portions of the terminal are caulked to the conductor portion placed on the bottom plate portion, a load acts also on the bottom plate portion and the caulking piece portions. The terminal is made of a highly-ductile sheet material such as a copper alloy sheet as described above, and the bottom plate portion and the caulking piece portions, when receiving a load, are extended in an axial direction of the conductor portion. In the aluminum wire press-clamping terminal disclosed in JP-A-2007-173215, the bottom plate portion 104 as well as the caulking piece portions 105 are uniform in thickness in the axial direction of the conductor portion 102, and the bottom plate portion 104 and the caulking piece portions 105, when receiving a load, are extended in the axial direction of the conductor portion 102, and are reduced in thickness. Particularly, a central portion (in the axial direction) of the bottom plate portion 104 and a central portion (in the axial direction) of each caulking piece portion 105 are extremely reduced in thickness. Therefore, it is feared that the strength of the terminal may be lowered, so that the press-clamping strength of the terminal may be lowered.